M. Hall

Theropod fauna from southern Australia indicates high polar diversity and climate-driven dinosaur provinciality.

The Early Cretaceous fauna of Victoria, Australia, provides unique data on the composition of high latitude southern hemisphere dinosaurs. We describe and review theropod dinosaur postcranial remains from the Aptian–Albian Otway and Strzelecki groups, based on at least 37 isolated bones, and more than 90 teeth from the Flat Rocks locality. Several specimens of medium- and large-bodied individuals (estimated up to ∼8.5 metres long) represent allosauroids. Tyrannosauroids are represented by elements indicating medium body sizes (∼3 metres long), likely including the holotype femur of Timimus hermani, and a single cervical vertebra represents a juvenile spinosaurid. Single specimens representing medium- and small-bodied theropods may be referrable to Ceratosauria, Ornithomimosauria, a basal coelurosaur, and at least three taxa within Maniraptora. Thus, nine theropod taxa may have been present. Alternatively, four distinct dorsal vertebrae indicate a minimum of four taxa. However, because most taxa are known from single bones, it is likely that small-bodied theropod diversity remains underestimated. The high abundance of allosauroids and basal coelurosaurs (including tyrannosauroids and possibly ornithomimosaurs), and the relative rarity of ceratosaurs, is strikingly dissimilar to penecontemporaneous dinosaur faunas of Africa and South America, which represent an arid, lower-latitude biome. Similarities between dinosaur faunas of Victoria and the northern continents concern the proportional representatation of higher clades, and may result from the prevailing temperate–polar climate of Australia, especially at high latitudes in Victoria, which is similar to the predominant warm–temperate climate of Laurasia, but distinct from the arid climate zone that covered extensive areas of Gondwana. Most dinosaur groups probably attained a near-cosmopolitan distribution in the Jurassic, prior to fragmentation of the Pangaean supercontinent, and some aspects of the hallmark ‘Gondwanan’ fauna of South America and Africa may therefore reflect climate-driven provinciality, not vicariant evolution driven by continental fragmentation. However, vicariance may still be detected at lower phylogenetic levels.

Cambrian metamorphic complexes in Tasmania: tectonic implications

Cambrian metamorphic complexes containing amphibolite‐ to eclogite‐grade rocks are present throughout western and northwestern Tasmania. These complexes contain mostly quartz‐albite‐biotite schists, garnet‐quartz‐albite‐biotite schists and mafic amphibolite lenses (up to 1 km long). The chemistry of these rocks is similar to unmetamorphosed, Late Neoproterozoic tholeiitic basalts and continental‐derived siliciclastics. A few rocks in the metamorphic complexes have compositions that are transitional between the amphibolites and the schists, representing metamorphosed volcaniclastic rocks formed by mixing between mafic and siliciclastic sources. The rocks in these complexes were probably located on the edge of a thin Late Neoproterozoic passive margin that was partially subducted during a Cambrian arc‐continent collision and uplifted during post‐collisional crustal re‐equilibration. The metamorphic condition, age and chemistry of both the schists and the amphibolites resemble those of metamorphic complexes in North Victoria Land in Antarctica. However, the structural setting of these complexes differs from those in Tasmania. Comparisons of the Tasmanian and North Victoria Land Cambrian structures and lithologies with those from more recent arc‐continent collisions worldwide show that both are compatible with a model involving east‐dipping subduction of a passive margin beneath an intraoceanic island arc. The differences between the two areas probably arise from differences in the geometry of the margins and the thickness of the passive‐margin sediments.

Reconstructing Rangea: New Discoveries from the Ediacaran of Southern Namibia

Abstract Rangea is the type genus of the Rangeomorpha, an extinct clade near the base of the evolutionary tree of large, complex organisms which prospered during the late Neoproterozoic. It represents an iconic Ediacaran taxon, but the relatively few specimens previously known significantly hindered an accurate reconstruction. Discovery of more than 100 specimens of Rangea in two gutter casts recovered from Farm Aar in southern Namibia significantly expands this data set, and the well preserved internal and external features on these specimens permit new interpretations of Rangea morphology and lifestyle. Internal structures of Rangea consist of a hexaradial axial bulb that passes into an axial stalk extending the length of the fossil. The axial bulb is typically filled with sediment, which becomes increasingly loosely packed and porous distally, with the end of the stalk typically preserved as an empty, cylindrical cone. This length of the axial structure forms the structural foundation for six vanes arranged radially around the axis, with each vane consisting of a bilaminar sheet composed of a repetitive pattern of elements exhibiting at least three orders of self-similar branching. Rangea was probably an epibenthic frond that rested upright on the sea bottom, and all known fossil specimens were transported prior to their final burial in storm deposits.

Fragmented Tasmania: the transition from Rodinia to Gondwana

The origin of the microcontinent VanDieland extends back to the late Paleoproterozoic, where it was positioned between East Antarctica and southwestern Laurentia, within the supercontinent Nuna and Rodinia. Paleo- to Mesoproterozoic events recorded in VanDieland have greater affinities with southwest Laurentia and East Antarctica, suggesting southern VanDieland was part of the Grenville Front, and the central Tasmanian part was adjacent to the Miller Range in the central Transantarctic Mountains. Late in the Neoproterozoic Rodinia break-up, VanDieland separated from East Antarctica and southwestern Laurentia, and moved north along the Terra Australis margin until its southern part was positioned next to the easternmost Robertson Bay Terrane of north Victoria Land. VanDieland comprises up to seven different crustal megaboudins or microcontinental ribbon terranes that likely had amalgamated by the end of the Cambrian; these ribbon terranes are bounded by major faults and suture zones. Some boundaries, such as the Arthur Metamorphic Complex, are well known. However, other boundaries, like the eastern edge of the Tyennan Zone, and the boundary between King Island and northwestern Tasmania, are more cryptic, as they are covered by younger geology or are under water. The boundaries are commonly defined by sedimentary and mafic volcanic infill that has been trapped between the crustal fragments. These rocks have previously been interpreted as allochthonous terranes but are more likely to represent inverted sections of attenuated transitional crust and back arc basin fill that formed along the eastern margin of the Gondwana plate during the Cambrian. This interpretation also provides an explanation for the previous tectonic analysis that suggests that Tasmanias mafic–ultramafic complexes were obducted westward onto older sequences and were subsequently transported southwards as other ribbons collided along the northeastern and western edges of the growing microcontinent, which existed in the overriding plate of a west-dipping subduction zone at the convergent margin between Gondwana and the proto-Pacific plate.

New Evidence on the Taphonomic Context of the Ediacaran Pteridinium

New material collected from the Kliphoek Member of the Nama Group (Kuibis Subgroup, Dabis Formation) on Farm Aar, southern Namibia, offers insights concerning the morphology of the Ediacaran organism Pteridinium. Pteridinium fossils previously described as being preserved in situ have been discovered in association with scour-and-fill structures indicative of transport. Additionally, two Pteridinium fossils have been found within sedimentary dish structures in the Kliphoek Member. A form of organic surface with a discrete membrane-like habit has also been recovered from Farm Aar, and specimens exist with both Pteridinium and membrane-like structures superimposed. The association between Pteridinium fossils and membrane-like structures suggests several possibilities. Pteridinium individuals may have been transported before burial along with fragments of microbial mat; alternately they may have been enclosed by an external membranous structure during life.

A new metazoan from the Vendian of the White Sea, Russia, with possible affinities to the Ascidians

Two specimens recovered from late Neoproterozoic shallow marine sediments of northern Russia may be the oldest known ascidians. Dated at around 555 Ma, these, together with the younger Ausia from the Nama Group in southwestern Africa, are probably relatives of these invertebrate chordates, which have a deep time origin predicted by molecular studies.

Stratigraphic architecture and depositional setting of the coarse‐grained Upper Cambrian Owen Conglomerate, West Coast Range, western Tasmania

The Upper Cambrian Owen Conglomerate of the West Coast Range, western Tasmania, comprises two upward‐fining successions of coarse‐grained siliciclastic rocks that exhibit a characteristic wedge‐shaped fill controlled by the basin‐margin fault system. Stratigraphy is defined by the informally named basal lower conglomerate member, middle sandstone member, middle conglomerate member and upper sandstone member. The lower conglomerate member has a gradational basal contact with underlying volcaniclastics of the Tyndall Group,while the upper sandstone member is largely conformable with overlying Gordon Group marine clastics and carbonates. The lower conglomerate member predominantly comprises high flow regime, coarse‐grained, alluvial‐slope channel successions, with prolonged channel bedload transport exhibited by the association of channel‐scour structures with upward‐fining packages of pebble, cobble and boulder conglomerate and sandstone, with abundant large‐scale cross‐beds derived from accretion in low‐sinuosity, multiply active braided‐channel complexes. While the dipslope of the basin is predominantly drained by west‐directed palaeoflow, intrabasinal faulting in the southern region of the basin led to stream capture and the subsequent development of axial through drainage patterns in the lower conglomerate member. The middle sandstone member is characterised by continued sandy alluvial slope deposition in the southern half of the basin, with pronounced west‐directed and local axial through drainage palaeoflow networks operating at the time. The middle sandstone member basin deepens considerably towards the north, where coarse‐grained alluvial‐slope deposits are replaced by coarse‐grained turbidites of thick submarine‐fan complexes. The middle conglomerate member comprises thickly bedded, coarse‐grained pebble and cobble conglomerate, deposited by a high flow regime fluvial system that focused deposition into a northern basin depocentre. An influx of volcanic detritus entered the middle conglomerate member basin via spatially restricted footwall‐derived fans on the western basin margin. Fluvial systems continued to operate during deposition of the upper sandstone member in the north of the basin, facilitated by multiply active, high flow regime channels, comprising thick, vertically stacked and upward‐fining, coarse‐grained conglomerate and sandstone deposits. The upper sandstone member in the south of the basin is characterised by extensive braid‐delta and fine‐grained nearshore deposits, with abundant bioturbation and pronounced bimodal palaeocurrent trends associated with tidal and nearshore reworking. An increase in base‐level in the Middle Ordovician culminated in marine transgression and subsequent deposition of Gordon Group clastics and carbonates.

Normal fault growth and its function on the control of sedimentation during basin formation: A case study from field exposures of the Upper Cambrian Owen Conglomerate, West Coast Range, western Tasmania, Australia

The evolution of a complex, ancient, segmented basin-margin fault system was examined using integrated structural relationships, lithostratigraphic architecture, facies architecture, and paleoflow trends to define the control on synrift development during the deposition of the Upper Cambrian Owen Conglomerate, West Coast Range, western Tasmania, Australia. Four distinct stages are recognized during the evolution of the normal fault array. The rift initiation stage defined early fault nucleation and isolated growth before fault interaction led to strain localization and the abandonment of antithetic faults. Renewed fault initiation led to further fault growth prior to the transition to rift climax stage, with the development of a throughgoing, linked array. Boundary faults propagated to near-maximum length prior to significant basin formation, whereas segment interaction clearly exerted a primary control on synrift distribution and paleogeography. Temporal fault evolution is marked by highly varied displacement patterns and pronounced lateral shifts in maximum synrift thickness accumulations. The case study presented here is compared with existing models of fault growth and provides a field example for the assessment of basin formation, particularly in polydeformed terranes, which can offer crucial high-resolution temporal data on fault system evolution, associated stratal architectures, and sediment dispersal patterns.

Detailed paleogeographic evolution of the Bass Basin: Late Cretaceous to present

The Bass Basin, southeastern Australia, is a moderately explored, Late Cretaceous to late Cenozoic, intra-cratonic rift system. Despite the success of hydrocarbon exploration in the adjacent Otway and Gippsland basins, attempts to locate significant hydrocarbon reservoirs in the Bass Basin have been disappointing, with only one major gas accumulation found in the centre of the basin since drilling began in 1965. However, owing to the basins tectonostratigraphic characteristics, which typically support the accumulation of significant reservoirs, the Bass Basin remains a viable target for hydrocarbon exploration. In this study, a detailed, fine-scale, stratigraphic framework, employing spore–pollen biozones as an age control of potentially suitable hydrocarbon source, reservoir and seal facies has been constructed to provide a better understanding of the basins paleoenvironmental evolution. Data from various sources, including well completion reports, drill core, cuttings descriptions and seismic sections were collated and analysed. Data from 18 wells were used in order to construct two cross-sections; one extending from the northwest to southeast along the basin and the other extending northeast to southwest. Up to 12 biozones were correlated between the wells. Key stratigraphic units and biozones were then correlated with hydrocarbon source, reservoir and seal and were traced along the basin. New results demonstrate that from the late Maastrichtian the depositional system was dominated by a large tidal inlet or lagoonal system situated between a marine influence towards the northwest and fluvial processes operating in the far southeast. Major hydrocarbon source facies are mainly located within the middle to upper Eocene, upper M. diversus to P. asperopolus biozones in the Eastern View Coal Measures and are generally concentrated in the northwest section of the basin. Potential reservoir and seal facies also appear to be thickest in this region. This new research reveals that from an exploration perspective, risk increases towards the southeast as the concentration of source facies diminishes, and the thickness of potential reservoir and seal facies decreases.

The variation of crustal stretching and different modes of rifting along the Australian southern continental margin

ABSTRACT The southern margin of Australia is a passive continental margin, formed during a Late Jurassic–Cretaceous rifting phase. The development of this passive margin is mainly associated with extensional processes that caused crustal thinning. In this work, we have measured the amount of extension and the stretching factor (β factor) across seven transect profiles approximately evenly distributed across the margin. The obtained results show that the amount of extension and the β factor along the margin vary from west to east. The lowest amount of extension, low–intermediate β factors and a very narrow margin are observed in the western part with 80 km of extension and is underlain mostly by the Archean Yilgarn Craton and the Albany–Fraser Orogen. The Gawler Craton in the centre of the south Australian margin is another region of low extension and low–intermediate β factor. The largest amount of extension (384 km) and the largest β factor (β = 1.88) are found in the eastern part of the passive margin in an area underlain by Phanerozoic Tasman Orogen units. Our results imply that there is a strong control of the age and thickness of the continental lithosphere on the style of rifting along the Australian passive margin. Rifting of old and cold lithosphere results in a narrow passive margin, with the formation of relatively few faults with relatively wide spacing, while rifting of younger, warmer lithosphere leads to wide rifting that is accommodated by a large number of faults with small spacing.